Currently, there is a trend in which roasters are measuring the water content (moisture) of their coffee. This is fine. This is good. It is certainly never a bad thing to know more about the coffee we are roasting. It provides some insight into how that green coffee will keep; how shelf stable it is. Although, I feel that this correlation is still in need of a lot of testing.

What information measuring moisture in coffee does not give a roaster, in my opinion, (and I would wager… my controversial opinion) is how to better roast coffee. Really, it tells us nothing concerning how a coffee will roast… at all.

Now, I know that at first, this sounds CRAZY. Bear with me.

The moisture that is in the coffee when it is green is free moisture, meaning it is not a molecule of potential H2O that is currently part of a larger complex molecule. It is simply free water in the seed. H2O that we see during first crack is mainly created during reactions (Maillard Reactions, Caramelization) from the disintegration of complex molecules that are giving off H2O in the form of steam.

Let’s move forward…

Moisture is a GREAT conductor, and while it is still present in the coffee, can work to wick heat into the core of the seed. Thus, its existence in the coffee is meaningful to how we heat the seed. But, the information a moisture meter gives us is being misused as a directive to how we should approach roasting a coffee.

The density of an object is simply Mass over Volume: D=M/V. We can measure this very easily for coffee by using a graduated cylinder. I recommend at least 250ml, but the bigger it is the more the more precise it will be. Fill it, and set it on a scale. There will be data noise with displacement (space between seeds) when measuring different screen sizes. The larger the tools, the less variable displacement exists, giving a more accurate reading. Some moisture meters come with a density tool, which is just awesome. Use it! That would be my preference, as it is much more accurate and easy.

Density is usually defined by: Grams/Milliliters, Grams/Cubic Centimeters, Pounds/Bushels, etc… This is important to know because there is no standard within our coffee industry currently as to what definition we assess coffee too. Maybe we should have that. I would love to see a standardization in our language here.

OK! Back to moisture.

Let’s do a hypothetical exercise: pretend with me for a moment.

There are two 100ml cylinders.

Each cylinder is filled to the 100ml line.

There are two different coffees.

Each coffee measures 10% moisture.

(So far everything looks identical… right? The inference many people would make right now would be to roast these two coffees similarly. BUT WAIT!! THERE IS MORE!!)

Let’s, for the sake of this hypothetical, say that each cylinder is filled with exactly 100 coffee seeds. (I know… nonsense, but we are pretending.)

When weighing the cylinders:

#1 weighs 100g

#2 weighs 90g

Now, we can see through this hypothetical, that even though our moisture meter is showing we have 10% moisture for the coffee that we are checking, the moisture meter is not telling us what that is 10% of. By measuring for density, we now see that in coffee #1, 10% shows that in 100ml of coffee there are 10g of water. We can see that in 100ml of coffee #2 there is only 9g of water. So, even though they have the same percentage of water, one coffee has more water than the other.

Furthermore, I would argue that this difference in moisture does not matter to how heat is transferred to the coffee. The density of the coffee DOES matter to how heat is transferred to the coffee.

The physics of heat exchange are such that: heat transfers more quickly (conducts) through more dense objects, and more slowly, (is insulated against) through less dense objects.*

Therefore, density is key.

Having water present adds to the density and conductivity of a seed (ability to absorb heat.) This is, of course, very important. However, if a roaster looks at the moisture percentage only, they are being led to believe something about the coffee that is incomplete. In order to qualify and quantify the percentage of moisture in coffee, a roaster must know what that percentage correlates to. In other words, a low-density coffee may have the same percentage of moisture as a high-density coffee.

Think about two water balloons. Balloon #1 is 5 inches in diameter and balloon #2 is 10 inches in diameter. If each is filled with 10% moisture, the 10-inch balloon has about 8 times more volume of water. (V=4/3πr3)

Another hypothetical:

Imagine weighing out batches of coffee.

Batch #1 stats: 100g, 10% moisture

Batch #2 weighs 100g, 12% moisture

How should a roaster approach roasting these? Well, according to the current philosophies, they would treat them differently based on that moisture reading. The school of thought is that for Batch #1 the roaster should start with a lower heat level, as the coffee seed will take on heat more quickly because it has less water.

Batch #2 is a lot more coffee that is a lot less dense but will be roasted as if the inverse is true, based on how we currently look at moisture for roast preparation.

When thinking in terms of differences in our moisture levels for specialty coffee, there is very little difference from one coffee to another. However, in terms of density, that difference is much more varied. A low grown robusta from India could potentially have the same moisture level as a very high grown coffee from Colombia. Yet, would we suggest that we roast these in the same way? No! Yet, that is what we as an industry are currently teaching.

While it is true that there are many other factors beyond density that determine how a coffee seed should be roasted, measuring density can lay the best groundwork for how to initially approach a coffee before that coffee has been fully profiled. It will give you a head start that is meaningful and helpful, far more important than measuring for moisture can.

This will also lead to thinking about batches not only in terms of weight but also volume. Volume is very important to how air will flow through the drum, and how much time each seed will be spending conductive time, seed to drum and seed to seed.

In my opinion, measuring moisture and using that measurement has created a correlation of information to results that is not truly a causation. I look forward to spelling out some of these correlations that are actually kind of true, but not fully true, so that we can be ready for the outliers, and basically be better at our jobs.

I am working on some research that (so far) supports what I am saying (or may completely trash it.) Stay tuned! I want a very large data set so it may take some time. Discussion can start now, however-let me know you think! If you want to join me in the research, and you have had good tools for measuring and collecting data, I would be happy to assign tasks and crowdsource. Thank you for taking the time to read this!

*This is being wrongly taught in many places within our industry. Many roasting courses teach that heat travels more SLOWLY through more dense substances, and more QUICKLY through less dense substances. This is not only wrong according to physics, but is a misunderstanding of how coffee seeds heat. I hope to address this more in-depth in coming blogs.

22 thoughts on “Coffee Roasting: Measuring What Matters”

Batch #1 stats: 100g, 10% moisture
Batch #2 weighs 100g, 12% moisture
How should a roaster approach roasting these? Well, according to the current philosophies, they would treat them differently based on that moisture reading. The school of thought is that for Batch #1 the roaster should start with a lower heat level, as the coffee seed will take on heat more quickly because it has less water.

Batch #2 is a lot more coffee that is a lot less dense, but will be roasted as if the inverse is true, based on how we currently look at moisture for roast preparation.”

Did we accidentally end up approaching these batches correctly though? If batch one is more dense, and we’ve decided to use a lower heat level, that heat should move more easily through the dense coffee. And if batch two is more coffee that is less dense, we would want to use more energy to adequately heat that larger mass of beans, right?

Houston, you nailed it! This is why making the correlation is not always bad. However, this correlation has led to us applying information wrongly. By measuring the actual thing that is making the actual difference in how a coffee heats, we can more accurately approach outliers!

Size is a difference that will affect roasting, for sure. Density, size, volume of coffee, temp when the coffee goes into the drum, and what kind of flavors/aromas are hiding in the coffee waiting to be unlocked all matter a great deal.

“The physics of heat exchange are such that: heat transfers more quickly (conducts) through more dense objects, and more slowly, (is insulted against) through less dense objects.* ”

Joe .. great article but is insulted supposed to be insulated ?? (victim of spell check)

Another analogy is how sound travels through media … Better through more dense objects (remember the story of Indians listening to railroad tracks with their ear to hear a train coming before they could hear it through the air?”

Royal Coffee´s Chris Kornman just posted this on density last week: http://royalcoffee.com/green-coffee-analytics-part-iv-density/. To my surprise (and joy ´cause I really REALLY like you guys write these kind of stuff!), while I was searching for his previous density standard. Anyway, haven´t read Chris´ post yet. But wanted to comment on yours first. Which is an excellent blog Joe, thanks!

Some random thoughts:

Very well noted about all these coffees with 10% moisture which are NOT the same.. washed, semi-washed, natural, screen sizes, the weight of the batch etc. Although to me, as sample roaster, it appears that coffees with lower moisture indeed develop faster, while considering start temp, development etc.

Spot on also for your comments on volume! 120 grams of a MTGB Brazil fit in my half-cut copper-style Probat dispenser (what´s the offical name of this thing anyway??), 120 grams of a rubbish Mandeling Grade 5 don´t fit! While moisture could be equal.

Perhaps a kind of leeway would be to consider the standard packaging per origin / per grade / per container i.e. regular Robusta´s mainly have 320 bags of 60kgs, Colombia and Bolivia the 275 or 285 70kgs, Centrals 285 of 69kgs, Brazils either 320 of 60 or 325 of 59kgs recently. And that moisture Monsooned Malabar monster just abt. 250 in 50kg bags. Says a lot, right!

My way of measuring (only Sinar 6060, no density or water activity meter so far..) is to ALWAYS measure the same amount, i.e. 165 grams, as per our main warehouse. (good get quite filled up with that Malabar though ;)). But while typing this previous sentence I immediately question myself that also here volume matters….
Anyway, I did some small research almost a year ago, on measuring the same coffee (Brazil NY2 S17/18) with different amounts: https://pbs.twimg.com/media/CU1p5hnUkAABzfA.jpg
Interesting right. There´s a bandwith of almost 10% around the average score, for outer weights 50 and 250 grams.

“I am working on some research that (so far) supports what I am saying (or may completely trash it.) Stay tuned! I want a very large data set, so it may take some time. Discussion can start now, however-let me know you think! If you want to join me in the research, and you have have good tools for measuring and collecting data, I would be happy to assign tasks and crowd source. Thank you for taking the time to read this!”

Really looking forward to this, and duly noted 😉
And hereby offering my services with the Sinar AP6060 and the Probat BRZ-5!

This is excellent. I’m a home roaster and would love to know the general approach professionals take to a bean of higher or lower density. I have only theoretically measured moisture pre/post-roast by weighing, but could easily use density measurements before to take a type of approach. I would also be interested in a mean and standard deviation for bean density is if that is out there. Thank you!

Thanks for your insight Joe! I find this density discussion really interesting. In order to keep the airspace of the roaster uniform in size, would you suggest charging beans volumetrically versus by weight? I guess it wouldn’t be terribly important if you approached each coffee differently, but theoretically, if you wanted a single profile for multiple beans, right? Thanks!

Jeremias, I do think that measuring things both by volume and weight is a great place to start, and and will give you a lot better idea of how to control your roasts. I don’t recommend only looking at volume.

Thanks for the reply Joe! I just rewatched your Mill City video on the drying phase in which you comment on density. Would you generally recommend lowering the charge temp on denser beans to minimize the exterior of the bean from developing too quickly?

Sorry for my late reply! I have put this to the side for a while. BACK!!

I do not recommend a lower charge temp. I recommend a higher charge temp with very low gas at charge, and then ramping gas back up to a lower setting. does that make sense? I like to push conductive energy on coffees with more density.

I recommend that you look at your end temp goals. If you shoot to be no more that ten degrees higher or ten degrees lower than that temp for your charge temp, that is usually a good place to start. In other words, if you are going to finish at 410F, your charge should not be lower than 400 or higher than 420. This is a general guideline… not a hard rule. I find it to be helpful!

Thanks for the insight, Joe! I’ve been following that rule unknowingly for a while. What’s the thinking behind not having it to 10 below? Just to have enough momentum and a large enough temperature gradient?

Interesting take on moisture percentages in green beans… Is that reflective of the US market?

I was taught to measure the moisture content to gain consistency in roasting – just to make sure the moisture levels were similar, otherwise the roasting profile that was programmed into the roaster could produce inconsistencies if there was too much variation in moisture content. Generally you would assume that greens from the same farm would be dried at the same rate, however, we found that some co-ops would mix a little…

Also wanted to mention the great job you did with Mill City on the coffee roasting series – I added all of the videos to my blog.

This has nothing to do with a specific market. What I am saying is that measuring moisture percentage tells you only about the water percentage of that coffee relative to that coffee, not relative to other coffees. Looking at percentage only forces the roaster to assume 10% moisture in one coffee is different than 12% in another coffee. However, when you look at density and moisture, it is possible that two coffees measure different relative moisture percentages in fact contain the same amount of total moisture.

Moisture per volume does not tell you moisture total weight or presence.

If I am 75% water and my son is 80% water, I weigh 190lbs. and he weighs 60lbs. which one of us has more moisture?

According to coffee roasters, he does… he is 80%, right? But, according to the reality of the world we live in, I have a LOT more moisture.